A. Field of the Invention
The present invention relates to magnetic recording media mounted on data storages for computers and such information processing apparatuses or on data storages, especially hard disk drives, for household appliances. The present invention relates also to the method of manufacturing the magnetic recording media.
B. Description of the Related Art
Recently, the storage capacities of data storage devices and the recording capacities of magnetic recording media (hereinafter referred to simply as the “magnetic disks”) used for the information processing apparatuses including computers have increased steadily in association with the increase of the data processed by the information processing apparatuses and the downsizing of the information processing apparatuses. In association with the storage capacity increase, the rotating speed of the magnetic disk has been increased more and more to improve the data transfer speed. Currently, some hard disk drives rotate the magnetic disks at 15,000 rpm.
The natural vibration of the magnetic disk caused by its rotation is hazardous for high-speed rotation of the magnetic disk. The vibration of the magnetic disk caused by its rotation prevents the flying ability of the magnetic head, greatly impairing the reliability of the hard disk drive. To obviate this problem, glass disks exhibiting high rigidity have been used recently for facilitating high-speed rotation. However, the glass disks are expensive at present, and the use of these expensive glass disks increases the total costs of the hard disk drives. Therefore, relatively inexpensive aluminum disks have been used to realize stable high-speed rotation.
The aluminum disks have been thickened from 0.8 mm to 1.0 mm, and even to 1.27 mm, to improve their rigidity and to confine their natural vibration within an acceptable range. Recently, an aluminum disk of 1.6 mm in thickness has been proposed.
Usually, the magnetic disk includes an aluminum substrate, a nonmagnetic metal undercoating layer on the aluminum substrate, a magnetic layer on the undercoating layer, and a protection layer on the magnetic layer. The undercoating layer, the magnetic layer, and the protection layer are formed by sputtering, chemical vapor deposition (CVD) and other such conventional methods for film deposition. A lubricant is coated on the laminate formed so far. The lubricant is coated in many cases by dip-coating due to the excellent mass-productivity of this technique and the simplicity of the equipment for the dip-coating. The dip-coating is conducted by first diluting a stock solution of a liquid lubricant to an appropriate concentration considering the desired film thickness with a solvent (such as an organic fluorocarbon solvent). The disk with the laminate formed thereon is dipped for a certain period of time in the liquid lubricant solution prepared as described above. Then, the disk is pulled out of the liquid lubricant solution at a constant speed to evaporate the solvent and to absorb the lubricant component remaining on the disk onto the protection layer. In this way, a thin lubricant layer is formed on the protection layer. Alternatively, spin-coating and spray-coating can be employed to form the lubricant layer.
Although the dip-coating method has been employed to form the lubricant layer, it has been found that a lot of lubricant localizes to, and remains on, the end face portion of the magnetic disk opposite to the direction to which the magnetic disk is pulled up, and the remaining lubricant migrates with elapse of time to the peripheral portions in the major surfaces of the magnetic disk. Referring now to FIG. 6, the arrow A indicates the direction, to which the magnetic disk has been pulled up, and the darkened regions are the regions, to which the lubricant has migrated.
As the magnetic disk is thicker, the absolute amount of the lubricant migrating to the peripheral portions of the major surfaces becomes larger, since a larger amount of the lubricant localizes to the end face of the magnetic disk in proportion to its area. The lubricant is liable to migrate to the slider of the magnetic head flying over the peripheral portions of the magnetic disk to which the lubricant has migrated. As a result, the flying of the magnetic head becomes unstable, and the unstable flying of the magnetic head adversely affects the magnetic parametric performances and the durability against head seek (the durability against swinging). Although the current typical flying height of the magnetic heads is around 10 nm, the flying height of the magnetic heads is expected to be lower in perpendicular magnetic recording systems in the future. The lubricant migration will pose serious problems on these magnetic heads flying at a lower height.
To date, the problem of lubricant migration has been intangible. The lubricant amount on the end face of the magnetic disk was not included in the control items, since the magnetic disks were as thin as 0.8 mm or less and the flying height of the magnetic heads was from 15 to 20 nm, which is relatively higher than the present flying height of the magnetic heads.
Even when the lubricant concentration is optimized, the solvent for diluting the lubricant is selected, and the pulling-up speed is optimized to prevent the lubricant migration from occurring, it is impossible at present to prevent lubricant migration completely.
Japanese Unexamined Laid Open Patent Application 2003-6849 and Japanese Examined Patent Application H07 (1995)-58545 disclose techniques in which a wiper or a buff is pressed onto the disk surface on which the lubricant is coated, in order to uniformly distribute the lubricant and to flatten the disk surface. However, the wiping treatment and the buffing treatment are applied only to the major surface of the magnetic disk, i.e., to the head flying region and the recording region, solely to improve the frictional performances such as the constant start stop (CSS) performance and to prevent the magnetic head from sticking to the magnetic disk. The wiping treatment and the buffing treatment have failed to obviate the problems of lubricant migration, in which the lubricant deposited on the end face of the magnetic disk migrates onto the major surfaces of the magnetic disk with the passage of time.
The present invention is directed at overcoming or at least reducing the effects of one or more of the problems set forth above.